1. Field of the Invention
The present invention relates to a self-diagnosis apparatus which performs a diagnosis of failure in a control unit provided with a pressure sensor, such as an exhaust gas recirculation (hereinafter referred to simply as an EGR) control unit and a purge control unit for an engine of an automobile, by use of an output signal of the pressure sensor.
2. Description of the Related Art
For example, a self-diagnosis apparatus is disclosed in Japanese Laid-Open Publication No. SHO 62-51746 and it is known as a self-diagnosis apparatus which is provided in a control unit having a pressure sensor and performs a failure diagnosis of the control unit, based on an output signal of the pressure sensor. The self-diagnosis apparatus, disclosed in this publication, relates to a self-diagnosis apparatus for an EGR control unit and performs a failure diagnosis of an EGR valve by means of a pressure sensor mounted on an intake pipe. This self-diagnosis apparatus also performs failure judgment of the pressure sensor itself, such as failure judgment of a broken line and a ground fault of a signal detection line of the pressure sensor, by judgment (judgment of upper and lower limit values) of the sensor output level.
In addition to the aforementioned EGR control unit, a purge control unit is known as a control unit using a pressure sensor. In this purge control unit, a pressure sensor is provided in a fuel tank where an evaporated fuel or fuel vapor is generated, and a failure diagnosis of the purge control unit is performed by checking for leakage of fuel vapor by means of the pressure sensor.
A self-diagnosis apparatus such as this is required to perform self-diagnosis with high reliability. Therefore, for this purpose, a measure to counter freezing of the pressure sensor is necessary so that a mistaken failure judgment is not made even at the time of the freezing of the pressure sensor, but such a measure has thus far been unsatisfactory.
That is, for example, the EGR control unit has the possibility that water gets into the intake pipe, and consequently, there is the possibility that water adheres around the pressure sensor attached to the intake pipe. If, with the condition water adhered, in a cold district a vehicle is left for a long period of time after the engine is stopped, then it will be predicted that the water on or around the pressure sensor will be likely to freeze.
Normally, a semiconductor-type pressure sensor has a strain gage at its pressure sensing portion for sensing an intake pressure by detecting a tensile force in this portion and then converting it into an electric signal. However, if water adheres in the vicinity of this strain gage, then the strain gage will come to be subjected to the tensile force as ice will be formed, and a change in the intake pipe pressure will not be transmitted. Consequently, there is the problem that there are some cases where the intake pressure can not be detected.
Also, the self-diagnosis apparatus needs to be constructed at a low cost. Therefore, it is necessary to fabricate the pressure sensor at a reduced cost. In a control unit originally provided with a pressure sensor, such as an EGR control unit, the use of such a pressure sensor is effective in reducing the overall cost of production.
However, the absolute accuracy required of the pressure sensor of the EGR control unit will be sufficient if it is such that a change in pressure can be detected when the EGR control unit is turned on and off, and consequently, if this sensor is evaluated from the aspect of failure judgment, then an inexpensive pressure sensor will tend to be employed in cases where a downgraded specification such as a drift in a sensor output is allowed. Therefore, if such a pressure sensor is directly used to detect breaks, ground faults and the like of the sensor detection signal line by means of the judgment of the upper and lower limit values of the sensor outputs (judgment where a value of judgment is set to a pressure value which does not actually exist), then a sensor output value of an actually existing pressure of the intake pipe will often exceed an allowable range of the aforementioned upper and lower limit values because the drift in the sensor output is large and the absolute accuracy is low. As a result, there are some cases where it becomes difficult to perform the failure judgment of the sensor itself with reliability.
Also, the purge control unit, described as another control unit, is used so that an evaporated fuel (purge gas) from the fuel tank is absorbed by a canister, then it is supplied to the engine for combustion without being discharged to the ambient atmosphere. In a self-diagnosis apparatus for this purge control unit, a pressure sensor is provided in a fuel tank for checking for leakage of evaporated fuel or purge gas, and the pressure of the purge gas is detected by this pressure sensor.
Even in such a self-diagnosis apparatus for a purge control unit, while the cost of manufacture has been reduced by utilizing the pressure sensor originally provided in the purge control unit, the same problem as with the case of the aforementioned EGR control unit occurs, because the pressure sensor, which is originally used in this purge control unit, similar to the pressure sensor originally used in the EGR control unit, is a pressure sensor whose absolute accuracy is rather low just enough to detect a relative pressure.